In order to prevent collapse and ensure safety during excavation of a tunnel, there has been disclosed a system that includes a plurality of optical fibers straightly installed to an extremely long steel pipe in its longitudinal direction, to monitor movement of the ground during the excavation by measuring strain behavior of the steel pipe by utilizing Brillouin frequency shifts caused by a strain applied to these optical fibers (see Non-Patent Document 1 for example).
There has also been disclosed a distributed optical fiber pressure sensor cable for measuring accurately a pressure value that is constituted with a middle core formed of a flexible tube; pressure measuring optical fibers helically wound around the middle core; axial strain compensating optical fibers embedded in the thickness of the middle core; temperature compensating optical fibers with an extra length, loosely fitted inside the middle core together with a tensile resistance body; and a flexible outermost layer covering the outside of the middle core (see Patent Document 1 for example).